An induction balanced metal detector, of the type used to locate coins, rings and other treasure buried in soil within a few feet of the surface, has a search head that houses a transmit coil and receive coil. The metal detector has circuitry that transmits a periodic signal to the transmit coil as the search head is manually swept over a ground surface to detect buried metal objects. When the transmit coil passes over a metal object, a signal is generated in the receive coil due to perturbations in the magnetic field which cause the AC inductive coupling between the transmit and receive coils to become unbalanced. These receive coil signals are responsive to target characteristics such as size, depth below the ground surface, orientation with respect to the search head, and type of metal. In order to provide the user with information about the target's characteristics, (e.g., to distinguish coins from nails), some metal detectors measure the phase angle between the transmitted signal and the received signal. This phase angle is typically displayed to the user as a number on an output device such as an analog meter or a liquid crystal display (LCD). Under ideal conditions this phase angle can provide the user with accurate information regarding the target.
However, in actual practice the phase angle information is materially affected by ground mineralization and can also be affected by the target's orientation with respect to the search head. Under either of these conditions a single sweep of the search head can, with conventional displays, produce multiple phase angle readings and thereby result in an indecipherable output. In response to this problem some metal detector designers have also provided an audio output of the received signal where the tone's frequency corresponds to the phase angle of the signal and the tone's volume corresponds to the signal strength. An example of the prior art which uses a numeric LCD to display phase angle information and which also has an audio output is Maulding, U.S. Pat. No. 4,868,910, assigned to the assignee of this invention. This recent prior art approach provides the user with more complete information regarding phase angle and signal strength, but suffers in that the information cannot be latched for careful analysis and it also depends upon the user's audio memory and ability to discern frequencies in order to determine the target's characteristics.